In case you’re one of the people who believe that Joen Biden was President for a few minutes between 12:00 and when Obama took the oath of office: you’re wrong.
Obama became President at 12:00, even without being sworn in. He was not allowed to “enter on the Execution of his Office” until he was sworn in, but he was the President.
Don’t believe me? Look at CNN, they got it right:
Several minutes apart.
Still don’t believe me? Google “David Rice Atchison” and get back to me.